Transparent coatings of poly(methyl methacrylate) homopolymers and copolymers are known and applied to thermoplastic substrates, particularly, polycarbonate substrates, to impart UV light stability to the finished article. Such coatings may function as carriers for a UV stabilizing agent to protect the underlying substrate as well as function to provide a nondegradable surface. These coatings may be scratch-resistant, but longer coating life is generally obtained with the nonscratch-resistant type. Desirable or essential properties for these methyl methacrylic coatings include high gloss, retention of gloss after weathering, high heat distortion temperature, high elongation at break, UV stability, and resistance to microcracking. Particularly, microcracking with aging and exposure to weather has been a serious problem in such coatings.
Polycarbonate film is particularly sensitive to UV light. Upon exposure to UV light, polycarbonate film will not only undergo the discoloration and surface damage associated with polycarbonate sheet, but within short time periods, will experience sharp decreases in impact strength. Polycarbonate film, having a thickness of 5 mil, will generally fail in 6-12 months of exposure to direct sunlight. A thickness of greater than 60 mil is generally required for polycarbonate to retain impact strength for more than two years of exposure.
In addition to special UV sensitivity, the added flexibility of polycarbonate film accelerates deterioration of the surface coating. Particularly, Microcracking is accelerated as flexibility of the substrate increases. Microcracking of the surface coating leads to reduced transparency, coating failure in the form of flaking and peeling, and accelerated UV degradation of the substrate.
Thus, the UV stabilization of polycarbonate film is a difficult and recognized problem in the art. Film, as opposed to the sheet of U.S. patent application Ser. No. 696,671, filed Jan. 30, 1985, now U.S. Pat. No. 4,666,779, incorporated herein by reference, undergoes rapid physical property digradation upon weathering.
Therefore, it is an object of the present invention to provide polycarbonate film with a methyl methacrylic coating, which has improved resistance to microcracking, yet maintains desirable heat distortion properties, transparency, high gloss, good retention of gloss after weathering, desirable elongation at break, and excellent UV stability.